The Third Generation Partnership Project (3GPP) Technical Specification Group (TSG) Radio Access Network (RAN) 1 wireless communication standards development organization is discussing a proposal for wireless communication networks having base stations that save energy, otherwise known as “green” base stations. According to the proposal, if there is no active user equipment (UE) in a cell area served by an enhanced NodeB (eNB), the base station transitions to an energy saving state. In this state, the eNB transmits only a synchronization signal, a Common Reference Symbol (CRS) and a Master Information Block (MIB). In LTE Release-8, the MIB is sent on the Physical Broadcast CHannel (PBCH), the MIB comprises a system frame number (SFN), downlink system bandwidth, a number of signaled downlink transmit antennas (or the number of CRS ports), and Physical Hybrid ARQ Channel (PHICH) configuration information. Also in LTE Release-8, the synchronization signal is sent on the last 2 symbols of the first slot of subframe 0 and 5 and the PBCH is sent on subframe 0 (each subframe comprises two slots, wherein each slot corresponds to a 0.5 milli-second). Thus when configured, the eNB transmits downlink information in only subframe 0 and subframe 5 wherein nothing (i.e., blank or Do Not Transmit or Discontinous transmit (DTX)) is transmitted in the other downlink subframes. Alternatively, the other downlink subframes may be configured as MBMS Single Frequency (MBSFN) subframes with CRS transmission only near the beginning of these subframes. When there is an active user terminal in the cell, the eNB transitions from the energy saving state to a fully active state wherein the eNB starts transmitting the signals such as System Information Block (SIB) information, CRS in other subframes, etc. This proposal however does not address how an eNB or base station in the energy saving state detects when a UE is attempting to register with or connect to the eNB.
It is known generally for a 3GPP LTE Release 8 UE to send a Random Access Channel (RACH) waveform to an eNB based on information obtained in a System Information Block (SIB) sent in a Physical Downlink Shared Channel (PDSCH) to connect to the base station. In this case, the UE processes a Physical Downlink Control Channel (PDCCH) based on Physical Broadcast Channel (PBCH) information to obtain a downlink (DL) grant identifying a System Information Block (SIB) including RACH configuration information. Alternatively, the LTE Rel-8 UE obtains the RACH configuration information from a UE specific Downlink Control Information (DCI) Format 1A grant of a PDCCH order and in this case the UE is already connected to the base station and hence the eNB can transmit on a unicast PDCCH addressed the specific UE.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.